Investigation of branched-chain amino acid metabolism in the isolated, perfused rat hindquarter will be convinced. The relationship between the concentration of leucine in the perfusate and the formation of alpha-ketoisocaproate will be examined at intervals during perfusion, using a gas chromatographic method for measuring the concentration of alpha-ketoisocaproate. Factors influencing formation of branched-chain keto-acids will be examined. Attention will be given primarily to endocrine and dietary treatments. Factors influencing the activity of phenylalanine hydroxylase in vivo will be examined. Evidence was reported that the reductase for the biopterin cofactor required by this enzyme is increased by glucagon treatment of animals. Rats will be treated with other hormones to determine whether these too may affect the activity of either the reductase or the hydroxylase. The enzyme activity will be measured in livers from animals at intervals after they are first injected with hormone to follow the time pattern of the response. Subsequently, treatments that are effective will be examined to determine the basis for the response, i.e. whether enzyme activation, synthesis or degradation may be influenced.